Newborns display a stronger brain response to pain when they're under stress, but it isn't reflected in their behavior, British researchers found.

For the study, the investigators monitored brain activity and facial expressions of 56 healthy newborns to assess their response to the pain of a medically necessary heel stick.

Those with higher levels of background stress -- as determined by heart rate and levels of a stress hormone in saliva -- had more brain activity in reaction to the pain. But that didn't seem to trigger a change in their behavior.

The study was published Nov. 30 in the journal Current Biology.

"When newborn babies experience a painful procedure, there is a reasonably well-coordinated increase in their brain activity and their behavioral responses, such as crying and grimacing," said researcher Laura Jones, of University College London.

"Babies who are stressed have a larger response in the brain following a painful procedure. But, for these babies, this greater brain activity is no longer matched by their behavior," she said in a journal news release.

Jones said the findings provide another reason to minimize both pain and stress when treating and caring for babies. Stressed babies may not seem to respond to pain, even though their brain is processing it.